The use of digital equipment to store, transmit and reproduce visible images, both moving and stationary, has become firmly established in such arts as television, telemetering, photography, facsimile transmission, printing and related arts. Samples of the image density are taken at spaced intervals and their densities are represented in digital form. Binary codes are most frequently employed because of their simplicity.
A well-known procedure is to scan the image along closely spaced lines in succession, optionally sampling the density at spaced intervals along each line and producing a binary code for each sample representing its density. The sequence of such codes, called a "binary line scan function", may be stored, transmitted or otherwise used in digital equipment to represent the line. However, in order to conserve digital storage capacity and to reduce the length of the codes for other reasons, workers have made use of the fact that when line scan functions are analyzed statistically they are found to contain substantial redundancy, also called "entropy". In recognition of this, prior workers have developed a technique called "run length coding". An equivalent line code consisting of a sequence of run length codes is produced for each line, the codes successively representing "black runs"and "white runs", each such code specifically representing the length of the corresponding run. A "run" as defined is an uninterrupted sequence of either black or white samples. A measurement is made of the frequencies of appearance of different run lengths in graphic material of the general type to be encoded. Codes of varying numbers of digits are then defined and assigned to run lengths on the basis of frequency with the lengths of greatest frequency having the shortest codes. The result is an equivalent line code characterized by a substantial but inherently limited reduction in redundancy.
In some prior facsimile systems an analog signal was derived from the optical scan and transmitted in like form, usually after amplification and frequently after modulation on a suitable carrier frequency. However, such systems had practical limitations that precluded their use on communication channels of the type envisioned by this invention at the speeds and with the resolution attainable by the latter. They also required substantial analog storage capacity in cases where storage of facsimile data was required. Both of these distinctions also apply to prior systems in which the analog signal was sampled at a rate determined by the desired resolution and directly converted for storage or transmission to a binary or other digital equivalent.
The inadequacies of these earlier systems may be illustrated with a specific example. Suppose it is desired to transmit an original copy 18 inches wide and 27 inches long in 15 minutes, with a resolution of 0.01 inch in both dimensions. This implies that the detector 11 must travel at a rate of 0.03 inch per second, and since the desired resolution requires a line spacing of 0.01 inch, three lines are scanned per second. In any digital system this resolution further requires 1800 samples per line. Therefore, the system must transmit 5400 samples per second, and if each sample is represented either as a "0" or a "1" (respectively representing white and black,) this means a transmitted bit rate of 5400 Hz. Also, to store the copy in binary code it would require a storage capacity of 4.86 million bits.
The bit rate imposes a definite requirement on the bandwidth of the communication channel. Some channels could not be employed for this reason. For example, A.T. & T. Co. Schedule 4B and Western Union Tel Co. Schedule F voiceband data transmission channels are designed for signals of 600 to 3000Hz. They provide a bandwidth of 2400 Hz. which can, by use of digital transmission techniques, transmit 4800 bits per second.
It is therefore evident that for a given resolution there are both storage capacity requirements and limitations imposed by the combination of the available bandwidth for transmission and the desired rate of producing copy (15 minutes in the above example). As a result, when the latter rate is prescribed certain transmission channels are seen to be inadequate.